Disruption and overexpression ofauxin response factor 8gene ofArabidopsisaffect hypocotyl elongation and root growth habit, indicating its possible involvement in auxin homeostasis in light condition.

Auxin response factor (ARF) family genes play a central role in controlling sensitivity to the plant hormone auxin. We characterized the function ofARF8 inArabidopsisby investigating a T-DNA insertion line (arf8-1) and overexpression lines (ARF8OX) ofARF8.arf8-1showed a long-hypocotyl phenotype in either white, blue, red or far-red light conditions, in contrast toARF8OX that displayed short hypocotyls in the light. Stronger and weaker apical dominance, and promotion and inhibition of lateral root formation were observed inarf8-1andARF8OX respectively. Sensitivity to auxin was unaltered inarf8-1hypocotyls with respect to growth inhibition caused by exogenously applied auxin and growth promotion induced by higher temperatures.ARF8expression was observed constitutively in shoot and root apexes, and was induced in the light condition in hypocotyls. Free IAA contents were approximately 30% reduced in light-grown hypocotyls ofARF8OX, but were similar between those ofarf8-1and wild type. Expression of the threeGH3genes was reduced inarf8-1and increased inARF8OX, indicating that they are targets of ARF8 transcriptional control. Because the three GH3 proteins may be involved in the conjugation of IAA as suggested by, and because two of the threeGH3genes are auxin inducible, ARF8 may control the free IAA level in a negative feedback fashion by regulatingGH3gene expression.ARFfamily genes seem to control both auxin sensitivity and homeostasis inArabidopsis. [ABSTRACT FROM AUTHOR]